Granulites of the Larba Block of the Dzhugdzhur-Stanovoi Superterrane: Reconstruction of the Formation Conditions
Sh.K. Baltybaev1,2, D.V. Dolivo-Dobrovolsky1, A.V. Yurchenko1, Yu.R. Volkova3, E.S. Malchushkin3
1Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
2St. Petersburg State University, Institute of Geosciences, St. Petersburg, Russia
3OSP Amurgeologiya AO Dal’nevostochnoe Proizvodstvenno-Geologicheskoe Ob’edinenie, Blagoveshchensk, Russia
Keywords: Granulites, P-T parameters, enderbites, age, metamorphism, Larba block, Aldan Shield
Abstract
We discuss the rocks of the Larba granulite block in the Ilikan zone of the Dzhugdzhur-Stanovoi superterrane. The Larba block is dominated by basic schists and garnet-biotite-orthopyroxene and garnet-biotite-cordierite-sillimanite gneisses (metabasites and metapelites). Calculation of temperatures and pressures of mineral formation was carried out by multi-equilibrium geothermobarometry, which makes it possible to evaluate the degree of equilibrium of mineral compositions along with P - T parameters. The P - T estimates have shown metamorphism of aluminous gneisses under moderate-pressure granulite facies conditions (7-8 kbar, 800-850 °C). Orthopyroxene granulites formed under granulite-amphibolite transition facies conditions. The mineral compositions and parageneses in highly ferrous metabasites permitted estimation of the conditions of metamorphism, P = 4-5 kbar and T = 630-700 °C, and show no influence of earlier granulite facies metamorphism. The bimodal P-T distribution for most samples of aluminous gneisses most probably reflects progressive and near-peak conditions of granulite metamorphism. The time of enderbite magmatism is determined from the upper intercept of discordia with concordia at 2546 ± 52 Ma and should be verified. The age of metamorphic rims over enderbite zircon is 1882 ± 11 Ma. The model Nd age of enderbites, t Nd(DM) = 2.57-2.58 Ga, is close to the age of the core of enderbite zircon and differs significantly from the model Nd age of the host metamorphic rocks (2.8-3.0 Ga). The Paleoproterozoic metamorphism of the Larba block rocks regionally coincides with the third stage of collisional granitoid magmatism of the southeastern Siberian craton and records the formation of this structure.
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